The surge won’t be uniform. Democrats will win big in more urban, more diverse, better-educated and more liberal-friendly states and will continue to lose ground in other states like Missouri. Although Mr. Trump may well win Ohio and perhaps even Florida again, it is not likely he will carry Michigan, Wisconsin and Pennsylvania in 2020. Look at the midterm performance of statewide Democrats in those states. And his troubles with swing voters, whom he won in 2016, will put Arizona, North Carolina and perhaps even Georgia in play for Democrats and effectively remove Virginia, Colorado, Nevada and New Hampshire from the list of swing states.

In short, the 2020 presidential election is shaping up as a battle of the bases, and the Democrats’ base is simply bigger. When their demographic advantage combines with an enthusiasm advantage and heightened party loyalty fueled by negative partisanship, they hold a significant structural advantage. Turnout in 2018 was about 12 points higher than 2014 turnout and higher than any midterm in decades. Midterm turnout can sometimes trail presidential-election-year turnout by 20 points. It was just 10 points in 2018, when it hit nearly 50 percent, versus 2016. It is not infeasible that turnout in 2020 will exceed 65 percent. Presidential-cycle electorates are better for Democrats than midterm electorates are, and the third-party share in 2018 was also at its lowest levels in decades. In congressional midterms, the average third-party balloting rate in recent elections is about 3.5 percent; in 2018, it was just half that.